A new website promises to shine a spotlight on "churnalism" by exposing the extent to which news articles have been directly copied from press releases.

The website, churnalism.com, created by charity the Media Standards Trust, allows readers to paste press releases into a "churn engine". It then compares the text with a constantly updated database of more than 3m articles. The results, which give articles a "churn rating", show the percentage of any given article that has been reproduced from publicity material.

The Guardian was given exclusive access to churnalism.com prior to launch. It revealed how all media organisations are at times simply republishing, verbatim, material sent to them by marketing companies and campaign groups.

Meanwhile, an independent film-maker, Chris Atkins, has revealed how he duped the BBC into running an entirely fictitious story about Downing Street's new cat to coincide with the site's launch.

The director created a Facebook page in the name of a fictitious character, "Tim Sutcliffe", who claimed the cat – which came from Battersea Cats Home – had belonged to his aunt Margaret. The story appeared in the Daily Mail and Metro, before receiving a prominent slot on BBC Radio 5 Live.

Atkins, who was not involved in creating churnalism.com, uses spoof stories to highlight the failure of journalists to corroborate stories. He was behind an infamous prank last year that led to the BBC running a news package on a hoax Youtube video purporting to show urban foxhunters.

The creation of churnalism.com is likely to unnerve overworked journalists and the press officers who feed them. "People don't realise how much churn they're being fed every day," said Martin Moore, director of the trust, which seeks to improve standards in news. "Hopefully this will be an eye-opener."

Moore said he accepted journalists often have a valid reason for using press releases, and will often need to copy and paste significant chunks, such as official statements and quotes. But he said that on many occasions reporters appear to be lifting press release text verbatim and adding little or no additional material.

In a typical example, the Express, Mirror and Sun all lifted of chunks of text from a press release last month on behalf of the Benenden Healthcare Society, which quoted a poll showing "British women spend more money on their looks than their health". The Daily Mail copied 98% of the text directly from the press release. Similarly, the Mirror, Mail and Express all reproduced chunks from a press release by campaign group Migration Watch, criticising immigration rates under the previous Labour government. However, the Times made the greatest use of it, running an article that was 53% cut-and-pasted press release.

Other broadsheets, including the Guardian, Independent and Telegraph, are also shown to be "churning" press releases to varying degrees.

Interestingly, all media outlets appear particularly susceptible to PR material disseminated by supermarkets: the Mail appears to have a particular appetite for publicity from Asda and Tesco, while the Guardian favours Waitrose releases.

Moore said one unexpected discovery has been that the BBC news website appears particularly prone to churning publicity material.

"Part of the reason is presumably because they feel a duty to put out so many government pronouncements," Moore said. "But the BBC also has a lot to produce in regions that the newspapers don't cover."

It was after a discussion with Moore that Atkins decided to spend last week seeding fictitious stories through the media. Atkins, who was the film-maker behind Starsuckers (a documentary that duped the tabloid press in 2009 with hoax stories), has planted a range of bizarre press releases across the media.

One involved the marketing of the "penazzle", supposedly the male answer to the female beauty trend in crystal designs known as "vajazzles", which appeared in the Sun and in a column by Guardian writer Suzanne Moore.

Another fake story about a "chastity garter", which contains a text message-sending microchip to alert a woman's partner if she is being unfaithful, became the "most read" story on the Daily Mail's website. It was then reproduced by media outlets across the world, including morning TV news in the United States.

But it was Atkins's bogus Facebook campaign calling for the return of the prime minister's new cat, Larry, that has been most successful. Within 24 hours of the site having been created, the Mail ran the story under the heading: "But that's my auntie's cat: Man's claim on 'stray' No 10 ratcatcher (… and there's a Facebook campaign to get him back)." It also appeared in the Metro, under the heading: "Dear David Cameron: You've stolen my aunt's cat, please return him."

But the entirely made up story received most attention on Gaby Logan's BBC 5 Live show, in a light-hearted slot before the 1 o'clock news.. Discussing the story with chief political correspondent John Pienaar, Logan joked that the dispute over ownership of Larry the Downing Street cat was likely to "run and run".